1. Field of the Invention
This invention relates in general to poly(phenylene sulfide) compositions, and more particularly, but not by way of limitation, to poly(phenylene sulfide) compositions suitable for encapsulating articles such as electronic components. This invention also relates to methods of forming poly(phenylene sulfide) compositions and articles formed of poly(phenylene sulfide) compositions.
2. Description of the Prior Art
Due to their excellent heat, chemical and electrical resistance, poly(phenylene sulfide) compositions are known to be useful for encapsulating articles such as electronic components. As encapsulating materials, poly(phenylene sulfide) compositions provide protection and electrical insulation.
The mechanical properties and electrical resistance of an encapsulating material are important. The material must be suitable for use in connection with encapsulation techniques that do not break or dislocate wires or other parts. In many applications, the electrical resistance of the encapsulating material must not be adversely affected by high temperature and/or high humidity. The encapsulating material must be able to withstand certain physical stresses, particularly stresses encountered during shipping, handling and assembling associated with the completed product.
When used in applications such as direct and indirect encapsulation of small electronic devices, it is critical for the rate of thermal expansion of the encapsulating material to be approximately the same as the rate of thermal expansion of the encapsulated component. A substantial difference between the rate of thermal expansion of the encapsulating material and the rate of thermal expansion of the encapsulated component can result in the formation of cracks in the encapsulating material and/or failure of the encapsulated component.
Although poly(phenylene sulfide) resins inherently have good mechanical properties and electrical resistance, their rate of thermal expansion is significantly different from the rate of thermal expansion of many components. In order to compensate for this difference, a non-reinforcing filler is typically admixed with the poly(phenylene sulfide) resin. The non-reinforcing filler makes the overall rate of thermal expansion of the encapsulating material approximately the same as the rate of thermal expansion of the encapsulated component. Unfortunately, the non-reinforcing filler also tends to lower the mechanical properties, particularly the tensile strength, of the encapsulating material. The reduction of mechanical properties makes it difficult to ship, handle and assemble the completed product and can adversely affect the product during use.
Although many additives are known to improve the tensile strength and mechanical properties of certain thermoplastic resins, there are no additives or additive systems that function to improve the tensile strength and other mechanical properties of all thermoplastic resins. The fact that a particular additive or additive system improves certain mechanical properties of one type of thermoplastic resin does not necessarily make it probable or even likely that the same additive or additive system will improve the same or additional physical properties of a different thermoplastic resin. Furthermore, even if a particular additive or additive system improves the tensile strength and other mechanical properties of the resin composition, it may render the composition unsuitable for use in encapsulating applications. For example, although reinforcing materials can be used to improve the tensile strength and other mechanical properties of poly(phenylene sulfide) compositions, they are not suitable for use in applications in which uniform thermal expansion is important. Unlike non-reinforcing fillers which expand uniformly in all directions, reinforcing fillers tend to have a low rate of thermal expansion in one direction and high rates of thermal expansion in the other directions. Nonuniform expansion of filler material can cause the encapsulating material to crack. Other additives or additive systems that may improve the mechanical properties of the resin often prevent use of the resin in certain encapsulation techniques.